In coating applications, especially automotive OEM (original equipment manufacturer) or refinish multi-coat systems, multiple layers of coating compositions are applied over metal substrates, such as an automotive body. Typically, the bare metal surface is coated with a wash primer, followed by a coating from a base coating composition, which is further coated with a clear coating composition. If desired, a coating from a primer composition may be applied over the wash primer coating before the application of the base coating composition. The adhesion strength and corrosion resistance of the coating resulting from the wash primer is important for the overall integrity of the multi-coat system. A conventional wash primer is a coating composition that typically includes a polyvinyl butyral dissolved in isopropyl alcohol. It typically also includes a corrosion inhibitor, such as zinc chromate, and an adhesion promoter, such as phosphoric acid that etches the metal surface to improve the adhesion of the primer to the underlying bare metal substrate surface. A layer of the wash primer is typically conventionally applied, such as by using a paint spray gun, to a typical film thickness of about 7.6 micrometers (0.3 mil) to 31 micrometers (1.2 mils), in contrast to a primer, which is typically applied to a film thickness of about 25.4 micrometers (1 mil) to 50.8 micrometers (2 mils) in OEM applications and up to 127 micrometers (5 mils) in refinish applications. Typically, a coating of a conventional primer coating composition is applied over the wash primer coating before additional coatings, such as base and clear coatings are applied.
One of the major problems associated with such conventional wash primers is the presence of the conventional corrosion inhibitor, such as zinc chromate, which is toxic and has an adverse impact on the environment. Thus, attempts have been made to produce chromate free wash primers that equal or improve the corrosion resistance provided by chromate-containing conventional wash primers.
The other problem associated with such conventional wash primers is the tendency of the coatings resulting therefrom to split when exposed to chipping action typically experienced by automotive coating systems.
The chromate free coating compositions of the present invention are not only benign to the environment but they also provide improved chip resistance over conventional wash primers.